The Universal Serial Bus (USB) is an industry standard interface that defines a method of connecting two units (usually a computer and a peripheral device) together and communicating between them. The USB interface on each unit contains a USB link controller (Link) and a USB physical interface circuit (PHY). To communicate, one of the units operates as a USB Host which initiates communication requests between the units, and the other operates as a USB Device which responds to those requests.
When the USB standard was developed, it was assumed that the Host and Device functions were separate and permanent, meaning that a Host unit could never function as a Device unit and vice versa.
In recent years, the USB standard was enhanced to add the possibility of a unit that can function in either Host or Device mode. That capability is known as USB On The Go or OTG. For OTG, additional circuitry is added to the Link and the PHY, an additional signal or signals are added between the PHYs, and a new protocol is defined by which the two units can automatically determine which one of them will operate in Host mode and which will operate in Device mode. This approach utilizes a different PHY, a different connector, and a different cable than that utilized by the standard USB technology.
Another approach to providing a unit with both Device and Host capability would be to provide multiple ports, including separate Host and Device ports. This approach has a disadvantage in that it requires multiple connectors, multiple ASIC pins and the like.
Needless to say, these approaches add overhead and increase the cost associated with devices that are implemented with both Host and Device functionality.